An image recording apparatus for forming an image on a recording sheet, such as an ink jet printer needs to precisely keep track of the position of a sheet end to form an image at any desired position on the plane (space) of the recording sheet. Particularly, there has been a need for more precisely keeping track of the position of a sheet end in borderless printing of a photo, etc., needed increasingly in recent years. That is, in the borderless printing, a predetermined margin is provided from the sheet end of a recording sheet, and a slightly wider print area is set than the area of the recording sheet, thereby forming an image with no blank at the sheet ends. At this time, since the print area which is set for the borderless printing includes an outside area of the recording sheet, ink is also ejected to the outside area of the recording sheet and is not supplied to image formation, which goes to waste and makes dirt the periphery of a platen for supporting the recording sheet, etc. For the image formed on the recording sheet, the surrounding image is lost. In contrast, it the position of a sheet end can be kept track of more precisely, the margin size can be decreased, and the area of the recording sheet and the set print area can be more matched with each other. Therefore, waste of ink, occurrence of peripheral contamination, loss of the surrounding image, and the like as described above can be suppressed.
To detect an end part of a recording sheet, a method of providing a moving arm at a midpoint in a conveying passage of a recording sheet, detecting fluctuation of the moving arm caused by contact with the conveyed recording sheet with an infrared sensor, and detecting the leading end of the recording sheet has been known. However, when the recording sheet is conveyed in a floating attitude from the conveying passage in a direction normal to the sheet plane, a timing of contacting with the arm varies, and the thickness and the material of the recording sheet, etc., have an influence on the detection accuracy. Therefore, it is difficult to improve the detection accuracy. Further, in the method, the sheet end in the direction orthogonal to the conveying direction of the recording sheet (side end) cannot be detected.
JP-A-7-215528 (especially, FIG. 10 of this reference) describes a configuration wherein a roller-shaped conductive electrode and a piezoelectric element group provided on a conveying passage of a recording sheet. The piezoelectric element group includes a plurality of piezoelectric elements facing the electrode and arranged along the sheet width direction and being independent of one another. In this configuration, when the conveyed recording sheet enters the space between the electrode and the piezoelectric element group, a signal is output from the piezoelectric element group in response to the width dimension of the recording sheet, and the position of a side end of the recording sheet can be detected.
However, in the configuration according to JP-A-7-215528, the piezoelectric element group including six piezoelectric elements is disclosed in an embodiment, and thus the position of a side end of the recording sheet can be detected in only six ways and the detection accuracy is low. Although the detection accuracy can be improved by increasing the number of arranged piezoelectric elements, a considerable number of minute piezoelectric elements independent of one another need to be provided to realize the detection accuracy at a level demanded in the borderless printing as described above; it is not realistic considering the labor and the cost at the manufacturing time.
Such circumstances is applied not only to the ink jet printer, but also to other printers such as a thermal printer, etc., and a copier and a facsimile machine. Further, in addition to a recording sheet to record an image, to detect the position of a sheet with an image recorded thereon in a scanner for reading an image and converting it into an electric signal, it is also necessary to precisely detect an end position of the sheet; similar circumstances to those described above exist.